The circuit breaker is filled with a good arc-quenching gas, e.g. sulfur hexafluoride, at a pressure of a few atmospheres. The circuit breaker is of the type which includes a so-called "thermal" volume in which the pressure is likely to increase considerably by virtue of its temperature rising when the circuit breaker is opened and an arc is struck between its arcing contacts. On the first zero crossing of the electric current, the gas expands and blasts the arc. Generally speaking, such circuit breakers also include a volume in which gas is mechanically compressed by a piston and is directed onto the arc to assist in extinguishing it. When low currents are to be interrupted (unloaded line) the mechanical blast effect is the major blast effect. Conversely, when high value currents are interrupted (e.g. short circuit currents) it is the thermal blast effect which is the major effect.
In such circuit breakers it is important for the actuating energy required to interrupt the circuit to be as small as possible. However, in most cases, the increase in pressure in the thermal volume has the effect of braking the movement of the circuit breaker moving assembly. Published German patent specification no. 2 358 368 describes a circuit breaker in which both sets of contacts are moving contacts (i.e. a double-acting circuit breaker) in which the arc energy is used both to assist in separating the contacts and to blast the arc.
In such a device, the energy required to actuate the circuit breaker is reduced, but this is to the detriment of arc blasting efficiency because of the high temperature to which the blast gas is raised. Preferred implementations of the present invention provide a thermal blast circuit breaker in which the pressure rise in the thermal chamber serves to reduce the energy required to operate the circuit breaker, while retaining proper arc extinguishing efficiency.
In order to do this, energy from the arc is used to assist in displacing the contacts relative to each other, but a portion of the arc energy is taken to compress a volume of cold gas without heating the gas, which compressed cold gas is used to blast the arc.